Molecular signaling involved in immune system activation against root-knot nematodes by bio-control agents in tomato plants

The expression of key defense genes was detected in roots and leaves of tomato plants until the 12th day after treatments with a mixture of beneficial bio-control agents (BCAs), as soil-drenches. The expression of the same genes was monitored in pretreated plants at the 3rd and 7th day since the inoculation with the root-knot nematode Meloidogyne incognita. Genes dependent on SA-signaling, such as the Pathogenesis Related Genes, PR1, PR3, and PR5, were systemically over-expressed at the earliest stages of BCA-root interaction. BCA pre-treatment primed plants against root-knot nematodes. The expression of PR-genes and of the gene encoding for the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO), which catalyzes the last step of ethylene biosynthesis, was systemically enhanced after nematode inoculation in primed plants. Defense related enzyme activities, such as endochitinase and glucanase, were higher in roots of BCA-treated than in those of untreated plants, as well. On the contrary, the expression of genes dependent on JA/ET-signaling, such as Jasmonate Ethylene Response Factor 3 (JERF3), did not increase after nematode inoculation in primed plants. The antioxidant system, as indicated by catalase gene expression and ascorbate peroxidase activity, was repressed in infected colonized roots. Therefore, Systemic Acquired Resistance (SAR), and not Induced Systemic Resistance (ISR), is proposed as the molecular signaling that is activated by BCA priming at the earliest stages of root-nematode interaction. Such BCA-induced activation of the plant immune system did not directly act against nematode motile juveniles penetrating and moving inside the roots. It resulted in a drastically decreased number of sedentary individuals and, then, in an augmented ability of the plants to contrast feeding site building by invasive juveniles.